>>20681
>The biggest problem however is certain chemicals which can't be filtered or boiled out, even distillation can't help.×
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Common rainwater in suburban or rural areas must be pretty free from large amount of chemicals, especially if your reservoir stand far from roads and such places. Although cleanness of barrels and roofs (if barrels are placed near to collect water) matter. But I guess tap water with old pipes not really better.

Distillation must remove most of common chemicals except very rare ones, light fraction would go into air, many heavy things may became salts or so. There is also freezing technique, but it is harder to do with full barrel even than boiling.

>How to get 200 l water out of a barrel, standing on the ground without a tap on it

A pump? I use it at dacha to empty barrels.